Low grade silver bearing manganese dioxide ores are classified as refractory because at least 80% of the silver present in the ore is complexed, or locked, within the manganese dioxide crystal lattice. Commercial production of silver from such ores by a conventional method such as cyanidization yields only approximately 15-20% of the available silver. Removal of the remaining 80-85% of the silver by conventional methods, such as smelting, is prohibitively expensive.
In such refractory ores, the manganese is in a +4 oxidation state, commonly referred to as Mn(IV) oxidation state; Mn(IV) oxides are solids which are insoluble in water and acid. In contrast, manganese in a +2 oxidation state (Mn(II)) is soluble in water and acids; therefore, if the magnesium dioxide in refractory ores, Mn(IV), is reduced to Mn(II), the solid crystal lattice is disrupted as Mn(II) goes into solution, and silver is released from the complex.
Oxidation and reduction of manganese has been described in a recent book entitled "Offprints from Metal Ions and Bacteria", copyright 1989, John Wiley & Sons, Inc.; specifically, Chapter 13, entitled "Mechanisms of Oxidation and Reduction of Manganese", by K. H. Nealson, R. A. Rosson and C. R. Myers, discloses that oxidation and reduction of manganese have been recognized as microbially catalyzed reactions since the turn of the century, although little is known about the biochemistry involved. Bacterially mediated manganese reduction is characterized as either indirect or direct reduction.
Indirect reduction of manganese by bacterial action occurs when through microbial metabolism, bacteria generate powerful manganese reducing chemicals such as hydrogen peroxide, sulfides, ferrous iron, and organic acids, thiols, phenols, and quinones. In contrast, direct reduction of manganese is cell-mediated, via electron-transport manganese reductase enzyme systems. As disclosed in Nealson et al., only three reports of such direct reductions exist. One study did not use pure cultures. The second study used a bacterium, GS-15, which has not yet been taxonomically classified. The third study identified the direct manganese reduction bacteria, MR-1, as a strain of Alteromonus putrefaciens. In none of the studies was the bacteria applied to a refractory manganiferous ore.
The following U.S. patents disclose processes to release precious metals such as gold, silver, and platinum from ores of two different types.
______________________________________ U.S. PAT. NO. INVENTORS ______________________________________ 4,740,243 Krebs-Yuill et al. 4,752,332 Wu et al. 4,765,827 Clough et al. ______________________________________
In all three patents, the ores used include a metal sulphide ore (such as but not limited to iron pyrite) which may or may not contain gold or other desirable metal, and a reducible manganese-silver ore. A typical process to release precious metals as disclosed by the cited patents includes the following steps. Water is added to a mixture of the two ores; the ore solution is maintained at a pH between 0.5 and 5.0 by periodic additions of sulfuric acid. Ferric iron is added, and the material is leached for periods ranging from weeks to months. The precious metals are then extracted by conventional methods such as cyanidization.
In other embodiments disclosed by the cited patents, the mixed ores are inoculated with the bacteria Thiobacillus ferroxidans. In all such embodiments, the bacteria reduce manganese indirectly, by microbial metabolism rather than directly, by a manganese reductase enzyme system. Specifically, T. ferroxidans bacteria oxidizes ferrous sulphide to ferric sulfate which in the presence of water forms sulfuric acid which solubilizes manganese chemically. Soluble manganese was toxic to the bacteria; therefore, careful control of the ratios of mixed ores was required. The bacteria disclosed in the referenced patents are strictly aerobic (requiring oxygen, in air, to grow), chemoautotrophic (using carbon dioxide as the sole source of carbon and deriving energy from the aerobic respiration of ferrous iron, sulfur, and metal sulfides), acidophilic (growing best at pH values at or near pH 2), mesophyllic (growing best over the temperature range of 10.degree. C. to 45.degree. C.), and sensitive to the presence of organic matter. Using the disclosed bacteria in a leaching process therefore requires maintenance of acid conditions (pH 2-pH 3), continuous provision of air, the presence of iron and/or sulphite ores, and a nutrient solution to promote growth when the bacteria is contacted with the ore.